An evolved universal mobile telecommunication system territorial radio access network (E-UTRAN for short) and an evolved packet core network (EPC for short) form an evolved packet system (EPS for short). Many services of operators are run in the 2G/3G circuit switching (CS for short) domain. To reuse the existing CS domain services in an EPS network, it is proposed to connect to the CS domain core network through the EPS network so that a user equipment (UE for short) is able to initiate and receive CS services in the EPS network and return to the 2G/3G network through the E-UTRAN to perform the CS services. The network architecture and processing method may be called circuit switching fallback (CSFB for short).
During the execution of the CSFB procedure, the UE executes the combined attach or combined tracking area update procedure in the E-UTRAN to register with the mobile switching center/visitor location register (MSC/VLR for short) of CS core network devices from the EPS network. In the process, a mobility management entity (MME for short) selects a proper MSC/VLR. At this time, according to commercial settings of the operator, the MME may select an MSC/VLR that has a public land mobile network identity (PLMN ID) different from that of the EPS network to which the UE is attached. Next, the UE executes the CSFB process to fall back to the 2G/3G network and access the PLMN where the MSC/VLR is located, thereby completing subsequent CS domain services.
One method for executing the CSFB process to fall back to the 2G/3G is as follows: An evolved base station (eNodeB for short) executes a packet switching handover (PS HO for short) procedure to select a proper target 2G/3G cell for the UE to complete CSFB processing, where the cell is marked with a cell identity (Cell ID), and the cell ID includes information about the PLMN to which the cell belongs, that is, the Cell ID includes the PLMN ID. However, the eNodeB is not able to perceive the PLMN ID of the MSC/VLR selected by the MME in the combined attach or combined tracking area update procedure. At this time, the PLMN ID of the target cell selected by the eNodeB may be different from the PLMN ID of the MSC/VLR selected by the MME so that the handover target 2G/3G cell selected for the UE by the eNodeB may belong to other PLMNs. In this case, when the UE camps to the target 2G/3G cell, as the PLMN is changed, the cell is served by another MSC/VLR. As a result, the UE needs to register with the new MSC/VLR by executing the location area update (Location Area Update, LAU for short) procedure before performing the CS service. In the case of being a called party/call termination, a roaming retry procedure further needs to be executed for this new MSC/VLR and the MSC/VLR previously registered by the UE to complete the called procedure. The additional location area update and roaming retry procedures increase the processing delay of the CSFB procedure, and reduces the service success rate and user experience.